Within the Terrestrial Radio Access Network (UTRAN) of the Universal Mobile Telecommunication System (UMTS) the Iub interface is located between the Radio Network Controller (RNC) and the base stations (Node B). For the Iub interface, the Asynchronous Transfer Mode (ATM) and ATM Adaptation Layer type 2 (AAL2) are used for transporting Frame Protocol (FP) PDUs (Protocol Data Units).
Ethernet is becoming a reasonable alternative to cell based transport infrastructure. In case of Ethernet the Internet Protocol (IP) is seen as the most feasible network layer. A typical way to transport Iub traffic over IP is to transport FP PDU frames over UDP/IP.
Due to existing legacy systems, interworking between cell based protocols, e.g. ATM, and packet based protocols, e.g. IP, based nodes is required, e.g. between an ATM based RNC and an IP based Node B (base station, BTS). This functionality may be either located in explicit nodes (Interworking Unit, IWU) or be part of other network elements, like e.g. the RNC and Node Bs themselves. For user plane traffic those nodes terminate the AAL2 layer on the ATM interface and then send the received FP frames over UDP/IP (User Datagram Protocol/Internet Protocol) to the BTS and vice versa.
As shown in FIG. 11, Frame protocol (FP) PDUs, which are larger than 45 octets, are segmented by the AAL2 SSSAR (Service Specific Transmission and Reassembly Sublayer) into multiple, up to 45 octets long CPS (Common Part Sublayer) SDUs (Service Data Units). Each of those CPS SDUs is given a CPS header, and the complete CPS PDUs are then multiplexed into ATM cells.
FIG. 12 shows the format of an AAL2 CPS packet in greater detail. A CPS packet header (CPS-PH) comprises a Channel Identifier (CID) of eight bits identifying a user channel out of 256 channels, a Length Indicator (LI) of six bits indicating the length of a packet payload, a User-to-User Indication (UUI) of five bits as an SSCS (Service Specific Convergence Sublayer) specific indication, and a Header Error Control (HEC) of five bits serving for error detection in the CPS-PH. CPS-INFO represents the CPS packet payload (CPS-PP) comprising 1 to 45 bytes. Thus, a CPS packet may comprise 4 to 48 bytes.
In order to use the transport resources efficiently, each AAL2 connection might be shaped individually on a CPS packet basis before multiplexing it into ATM cells. Without shaping, the CPS packets are transmitted back-to-back as a burst whereas the shaping allows the transmission of the segments (i.e. CPS packets) with a finite inter-departure time. During the inter-departure time the CPS packets of other AAL2 connections using the same path may use the path capacity.
A node which is terminating the AAL2 layer and further processes the received AAL2 SDU normally has to wait until it has received all CPS packets for a complete AAL2 SDU. This is called “Store-and-forward” method, because first the complete AAL2 SDU must be received before it can be forwarded to the next processing layer. The time needed until the complete AAL2 SDU has been received depends on the shaping parameters, transport capacities and the size of the AAL2 SDUs.
Due to this “store-and-forward” method the delay imposed by an intermediate ATM/IP Interworking Unit (IWU) between the RNC and BTS, will be significantly increased, especially in case of large FP frames like for example those typical for HSDPA (High Speed Downlink Packet Access).
Another critical aspect for Iub is not only the total delay of the transport layer, but even more the delay variation the transport layer imposes. The frame protocol layer used on the Iub has the capability to adapt to the transport delay between RNC and BTS, but it still requires that the total delay remains within a given range. Thus, any larger variation will cause data losses. With ATM the delay variation remains small and is controllable, however, with a packet based transport network with intermediate nodes the delay is depending on the packet size, number of nodes, link capacity between the nodes and how large other packets are when they are blocking the link. Thus, the delay variation can become quite large.
Another drawback is that on the IP interfaces large UDP payload (more than 1472 bytes) is problematic, since due to the limited MTU (maximum transfer unit) size of Ethernet (1500 bytes) IP segmentation is required, which is a quite complicated and memory demanding functionality on the receiver side. In general, large packets are not recommended when the transport network consists of several nodes (e.g. IP routers, Ethernet switches) and low capacity links, because at each node the complete packet has to be received before it can be forwarded.
In addition, in case of small FP frames (e.g. voice frames), the overhead/payload ratio is poor if only a single small frame is transported within an IP packet, thus transport bandwidth will be wasted.
A prior art solution allows to multiplex small packets called “CIP” of variable size in one CIP container, also of variable size. This may allow an efficient use of the transport bandwidth by amortizing the IP/UDP overhead over several CIP packets.
A segmentation/re-assembly mechanism allows to split large FP PDUs in smaller segments, which will be the CIP packets. There has to be a trade-off between efficiency (IP header/payload ratio) and transmission delay. Large data packets have to be segmented in order to avoid IP fragmentation and to keep transmission delays low.
However, this solution assumes that the complete FP PDU is received before the frame is split into smaller segments.